The present application relates in general to the field of telecommunications and data transmission. More specifically, the present invention relates to a method, system and apparatus for acquiring frame synchronization and re-synchronization over a data communication system using pulse stuffing. The time to acquire initial frame synchronization is reduced by approximately a factor of two and the time for resynchronization is also substantially reduced. The present invention is particularly useful for reducing the amount of data lost during frame synchronization and resynchronization operations.
Conventional data communication systems typically transfer data in groups or blocks of words commonly referred to as data xe2x80x9cframes.xe2x80x9d Each frame is a cyclic set of consecutive time slots having a structure that defines the relative position of each time slot and the transmitted or xe2x80x9cpayloadxe2x80x9d data stream contained therein. In addition to the payload data, frames contain overhead bits whose function is typically to ensure synchronization, error detection and operation and maintenance, etc. However, the greater the ratio of overhead bits to data bits the lower the efficiency of the corresponding communications channel. Frames ordinarily must be synchronized in time in order for a receiver to properly reconstruct a data stream transmitted over a communications channel.
Frame synchronization (xe2x80x9csyncxe2x80x9d) or resynchronization (xe2x80x9cresyncxe2x80x9d) is typically accomplished by designating a portion of each data frame as a xe2x80x9cframe sync wordxe2x80x9d (FSW). Typically, the FSW is a pattern of bits placed at the beginning of each data frame that functions as a reference point or marker for a receiver processing data transmitted over a data communications channel. The FSW is usually combined with the data stream at the transmitter to form the data frames, which data frames are then modulated, transmitted over a data communications channel, and demodulated at a receiver. The demodulated signal is a data stream containing the FSW. The FSW is then detected by a deframer using an FSW detector in order to acquire frame synchronization. Optionally, a scrambler is provided in the transmitter for randomizing data along with a corresponding descrambler in the receiver.
Characteristics of frame sync words have been extensively studied and are discussed in textbooks such as Bernard Sklar, Digital Communications Fundamentals and Applications, pp. 460-464 (Prentice Hall 1988), and J. J. Spilker, Jr., Digital Communications by Satellite, pp. 449-454 (Prentice Hall 1977). An FSW must have good synchronization properties in order to ensure reliable data transfer over a data communications channel. An FSW has good sync properties when the FSW have small xe2x80x9ccorrelation sidelobes,xe2x80x9d which usually requires a certain number of dedicated xe2x80x9coverheadxe2x80x9d bits to represent the FSW information. The above-cited references have tables containing FSW""s with good properties and also provide examples showing how to determine the correlation sidelobes for a sequence of bits.
However, using too many overhead bits is often undesirable especially in data communication systems having limited bandwidth. A greater number of overhead bits can add undesired complexity to both the framing and deframing operations in the transmitter and receiver, respectively, and reduce the efficiency of the communication system. Therefore, although it is generally accepted that a well designed xe2x80x9clongerxe2x80x9d FSW has better sync properties than a shorter FSW, e.g., a 14 bit sync word as compared to a 10 bit sync word, it is often desirable to use the shorter FSW in order to reduce the complexity and cost of the system. Thus, there is a need to limit the number of overhead bits used for frame synchronization and resynchronization without compromising frame sync or resync performance.
The aforedescribed limitations and inadequacies of conventional methods and systems for frame synchronization and resynchronization are substantially overcome by the present invention, in which a principal object is reduce the time for frame synchronization and frame resynchronization without adding overhead bits to a data frame. This principal object is to be satisfied without compromising frame sync and resync performance.
Another object of the present invention is to provide a data communication system that has a robust sync and resync performance without significantly increasing the complexity of the framing and deframing operations in the corresponding transmitters and receivers.
The above and other objects are satisfied by a method, system and apparatus that provides a long frame sync word (LFSW) by uniquely combining a conventional frame sync word with stuff bits from the immediately preceding data frame.
In a first aspect of the present invention, a method is provided for acquiring frame synchronization or resynchronization in a system communicating data frames between a transmitter and a receiver. The method includes the steps of: generating a first frame sync word located at the beginning of a first data frame; appending payload and overhead bits to the end of the first frame sync word; appending stuff bits to the end of the payload and overhead bits, wherein the stuff bits function to correct timing offsets; generating a second frame sync word located at the beginning of the a second data frame; detecting a long frame sync word formed by the combination of the stuff bits and the second frame sync word; and acquiring frame synchronization based at least in part upon the detected long frame sync word.
In accordance with another aspect of the invention, the step of appending stuff bits further includes the step of appending a sequence of stuff bits having a complementary sync characteristic when combined with the second frame sync word. Using such a stuff bit sequence yields a long frame sync word having an enhanced frame sync property which in turn significantly reduces the mean time-to-sync of the receiver.
In yet another aspect of the present invention, a system is provided for acquiring frame synchronization of data frames transmitted over a data communications channel. The system includes: a framer for providing a first frame sync word at the beginning of a first data frame, for attaching a sequence of stuff bits at the end of the first data frame to correct timing offsets, and for providing a second frame sync word at the beginning of a second data frame; and a deframer for detecting the occurrence of stuff bits followed by the second frame sync word, whereby the combination of the stuff bits and the second frame sync word forms a long frame sync word having an enhanced frame synchronization property.
In still another aspect of the present invention, a transmitter apparatus for generating a sequence of data frames wherein the sequence contains frame sync words and long frame sync words is also provided. The apparatus includes: means for detecting timing offsets between a transmitter clock and a receiver clock, the detecting means providing a pulse stuff signal; and a framer for inserting a frame sync word at the beginning of each frame and for inserting sync compatible stuff bits at the end of selected frames in response to the pulse stuff signal. A receiver apparatus is also provided having: a means for detecting a long frame synchronization word; a means for detecting a frame synchronization word; and a circuit means for acquiring frame synchronization in response to the output of the two detecting means, wherein the circuit means is an arrangement of digital circuit elements programmed according to a frame synchronization algorithm.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.